In a pixel-based scanner, printer, copier, or fax machine, a swath consists of a number of pixel lines, modernly on the order of twenty to two hundred. In a swath scanning system the pixel columns within each swath are serially acquired, one pixel column at a time, by an imager and detector array which move together across the document on a carriage.
I call this type of operation "carriage scanning", "swath scanning", or "swath-type scanning"--and the devices "carriage scanners", "swath scanners", or "swath-type scanners". They are also sometimes called "moving scanners".
The ability of any scanning system to reproduce a document is largely dependent on the performance of its imager. The typical imager used in a document scanning system is purely refractive, aside from a planar folding mirror, and usually consists of a simple single lens or in some cases a compound lens system. Several optical characteristics useful in evaluating an imager's performance are set forth below.
(a) Focus and resolution--Imager focus varies as a function of object location, axially, within the imager's depth of field. A typical refractive imager has excellent focus near the center of the depth of field which decreases to merely acceptable focus behind and in front of that best-focus position, or in other words, near the ends of the depth of field.
Closely associated with focus is resolution. Resolution is a quantitative measure of an optical system's ability to produce a sharp image of an object. Imager resolution is affected by the quality of focus and, as will be seen, optical aberrations such as chromatic aberration. An imager with poor focus has poor or coarse resolution; an imager with good focus has good or fine resolution.
When points of an object are at different locations within a typical refractive imager's depth of field, i.e. at different axial positions, the object rays from points near the center of the depth of field are better focused in the resulting image than points farther from the center of the field. In scanning, variations in depth of field location arise when the document being scanned is wrinkled or misfed. The result is an image with varying focus--it is defocused at points where wrinkling has displaced the document from the best focal position.
Such a defect is particularly conspicuous when the document carries lines or edges angled very shallowly to the pixel grid so that serious aliasing occurs. Aliasing is smoothed out in areas that are defocused but appears plainly where the document is in sharp focus--an appearance that is both unpleasant and baffling to the casual user.
Therefore, it is disadvantageous to use a conventional scanner imager that produces images with variable resolution, dependent on axial displacement of the object within the imager's depth of field.
(b) Distortion--Distortion arises from variation of magnification due to (1) off-axis distance of an object point in a document, and (2) axial displacement of the document. A typical refractive imager is subject to significant distortion from both of these causes.
Variation of magnification with axial position arises because the primary ray from each object point is angled to the optical axis. Therefore fluttering or crumpling of a document, displacing any small region of the document in and out along the optical path, changes the off-axis distance of the intercept of the angled primary ray with the document surface. This change in off-axis distance amounts, for that region of the document, to variation in magnification--i.e., distortion.
It is thus disadvantageous to use a conventional scanner imager that produces images which are distorted when documents are wrinkled, dependent upon both off-axis distance and axial displacement of an object.
(c) Brightness--Image brightness varies as a function of object off-axis distance. A typical refractive imager collects more light from object points closer to the imager's optical axis than from points farther away from the axis.
The result is a swath image with a bright midregion and relatively dark extremes, producing a mottled or dirty effect. It is therefore disadvantageous to use a conventional scanner imager that collects light nonuniformly, dependent upon object off-axis distance.
(d) Chromatic aberration--In a typical refractive imager, chromatic aberration occurs when an object ray of white light refracts through the optical material of the imager. This refraction causes rays of various wavelengths to leave the material at different angles (spectral dispersion) and come into focus as points of different colors at different locations.
The result is a blurred image. The amount of blur is greater for object points farther from the optical axis because their rays enter the imager at its extremes, where refraction is typically greater--thus again creating a patchwork effect, or an overall impression of variable quality in finished copies. Accordingly, it is disadvantageous to use a conventional scanner imager with significant chromatic aberration.
(e) Banding--The above-discussed effects, in a swath scanner, both individually or collectively result in undesirable conspicuous banding. This occurs as follows.
Suppose an imager is called upon to scan a document composed of a series of closely spaced horizonal lines. Further suppose the imager has nonuniform brightness, chromatic aberration, and expansive distortion. Under these conditions each swath produced by the imager has dark, blurred, expansively spaced lines at the top and bottom of the swath, and bright, clear, narrowly spaced lines in the midregion of the swath.
Upon reassembly of the swaths a banding effect is apparent. The document image is a cyclic display of dark, blurred, expansively spaced groups of lines--alternating with bright, clear, narrowly spaced groups of lines.
The magnitude of these effects depends on the size of the swath. The bigger the swath, the farther points are from the imager's optical axis; accordingly, there is greater distortion at the extremes, greater disparity between the brightness of the swath center and the swath edge, and greater blur at the extremes.
(f) Inefficient light collection--An imager having inefficient light collection capability does not collect adequate light from the object. This results in an image with poor contrast.
In a scanning system, sufficient light must be collected from each pixel being scanned. This is particularly important in a swath scanning system because the rate at which each pixel is acquired is proportionately higher relative to a full transverse scanner. Therefore, it is disadvantageous to use a conventional scanner imager that cannot collect enough light from each pixel during the shorter exposure times needed for swath scanning.
(g) Related devices--As previously mentioned, a typical scanning system employs, as its imager, a single refractive lens or a compound lens system. A single lens does not possess the desired characteristics. The image quality of a single lens is strongly dependent on both off-axis distance and axial displacement of an object.
A complex, compound refractive lens system has some of the desired characteristics because it can correct for some off-axis distance effects such as distortion and chromatic aberration. This type of system, however, is moderately expensive and delicate. Considering the commercial and industrial environments in which these imagers must operate, it is disadvantageous to use an imager that is at once so costly and so touchy.
Besides the imager, other elements of a scanning system may compensate for poor imager performance. For example, the detector array may be calibrated to compensate for brightness variation. Compensation for inefficient light collection may be obtained by slowing down the scanning rate, thereby increasing exposure time, or by increasing the level of object illumination.
These solutions, while compensating for the inadequacies of the imager, create other problems. A calibrated detector array requires application of a weighting factor for each row in a swath, and a sizable calibration can use up some of the effective dynamic range--thereby degrading effective signal-to-noise ratio. Slower scanning speed results in a less efficient scanner, and greater illumination requires more operating power.
(h) Previously unrelated devices--Not previously associated with or suggested for use in scanners, are certain devices which have useful optical properties. Dating from Isaac Newton's astronomical telescope, it has been known to use nonrefracting, or reflecting, elements in imaging devices. Benefits of simple reflecting imagers include reduced chromatic aberration and increased light efficiency.
More modern imaging devices, such as the Dyson catadioptric imager and the Offner catoptric imager, have further useful optical properties. The Dyson imager has low distortion (near uniform magnification); however, it cannot correct for poor resolution due to axial displacement of an object within the imager's depth of field.
The original Offner imager, never suggested for use in a scanner, combines favorable features of reflecting imagers and the Dyson imager. It can accommodate axial displacement of an object in terms of magnification uniformity, but not in terms of resolution uniformity. In other words, the original Offner imager cannot produce an image with uniform resolution within its depth of field.
As previously stated, none of these imaging devices has been associated with, or suggested for use in, document scanning systems.
(i) Conclusion--Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.